Snapshot test function improvements and more snapshot tests!

[vezwork]

• add warning log when creating a snapshot
• add throw if attempting to create a snapshot in CI (thank you @cscheid)
• add roundtrip snapshot test helper function roundtripSnapshotTest
• add roundtripSnapshotTest('valid-basics-2.qmd')
  • this test covers many more quarto features that can be inserted by the visual editor such as tables, citation, crossref, block quote, line block, escaped code blocks, raw blocks, html comments, definitions lists, horizontal lines, fenced divs, and footnotes
  • valid-basics-2.qmd comes with a references.bib file which is not necessary for the tests. I included it to have a more legit Quarto document setup... maybe it can be used in future tests.
• add roundtripSnapshotTest('valid-nesting.qmd')
  • this test covers some ways that things can be nested in Quarto, especially code blocks and quote blocks
• remove roundtrip-changes.qmd test because it is now covered by snapshot tests

I'm motivated to add these tests in order to get enough roundtripping coverage to feel confident about merging my refactor of some code around converting from qmd to visual editor #779